Method for recovering performance of electrolyzer for use in production of polysulfide and method for stopping  holding electrolyzer

ABSTRACT

In an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, wherein the anode compartment of the electrolyzer is cleaned with the use, of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent, thereby recovering performance of the electrolyzer. Further, when the contents of the anode compartment are replaced with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation, thereby maintaining the performance of the electrolyzer.

FIELD OF THE INVENTION

The present invention relates to a method for recovering performance of an electrolyzer for use in production of polysulfide by electrolytic oxidation of a white liquor or a green liquor, a method for stopping-holding an electrolyzer and a method for efficiently producing polysulfide by making use of the method for recovering the performance of the electrolyzer and the method for stopping-holding the electrolyzer.

BACKGROUND OF THE INVENTION

To attain a higher production yield at the time of production of chemical pulp is an important issue for effective utilization of wood resources. There is available a polysulfide cooking process as one of high yield technologies for production of kraft pulp currently in the mainstream of the chemical pulp. As disclosed in, for example, JP 61-259754 A, and JP 53-92981 A, a digesting liquor in the polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium sulfide, the so-called white liquor, in the presence of catalyst, such as activated carbon, and so forth, by the agency of molecular oxygen such as air, and so forth, as expressed by a reaction formula (1) given hereunder.

With this method, it is possible to obtain a polysulfide cooking liquor of polysulfide sulfur concentration around 5 g/L at an oxidation rate (Consumed Na2S (as S) in WL/Na2S (as S) in original WL) on the order of 60%, and a selectivity (PPS (as S) produced/Consumed Na2S (as S) in WL) on the order of 60% on a sulfide ion basis. With this method, however, if the oxidation rate is raised, many thiosulfate ions making no contribution to digestion at all are produced as a by-product due to secondary reactions (side reaction) as expressed by reaction formulas (2), (3) given hereunder, so that it has been difficult to produce a digesting liquor containing polysulfide sulfur in high concentration at a high selectivity.

4 Na₂S+O₂+2 H₂O→2 Na₂S₂+4 NaOH   (1)

2 Na₂S+2O₂+H₂O→Na₂S₂O₃+2 NaOH   (2)

2 Na₂S₂+3 O₂→2 Na₂S₂O₃   (3)

Herein, polysulfide sulfur is also referred to as (PS—S), and to sulfur having a valence 0 in, for example, sodium polysulfide Na₂S_(x), that is, sulfur corresponding to (x−1) atoms. Further, sulfur corresponding to sulfur of oxidation number—2, in polysulfide ions (sulfur corresponding to one atom per Sx²⁻ or Na₂S_(x)), and a sulfide ion (S²⁻) are generically referred to as Na₂S mode sulfur in the present specification. Further, the unit of volume liter is expressed “L” in the present specification.

Meanwhile, in JP 11-343106 A, there has been disclosed a method for electrolytic production of a polysulfide cooking liquor. This method is a polysulfide production method whereby a solution containing sulfide ions is introduced into an anode compartment of an electrolyzer, the electrolyzer comprising the anode compartment provided with porous anodes, each anode having a physically continuous three-dimensional mesh structure, at least a surface thereof being made of nickel, or a nickel alloy containing not less than 50mass % of nickel, and having a surface area 500 to 20000 m²/m³ per unit volume of the anode compartment, a cathode compartment provided with cathodes, and a membrane providing a partition between the anode compartment and the cathode compartment, thereby obtaining polysulfide through electrolytic oxidation.

It is accumulation of impurities originating from the solution used as raw material that poses a problem with stable continuation in operation of the electrolyzer over the long term. It is known that those impurities include, for example, Ca, Sr, SO₄, I/Ba composite, Al/SiO₂ composite, Mg, Ni, and so forth. Those impurities are deposited on the membrane to cause deterioration in current efficiency, or to cause an increase in membrane resistance, thereby inducing a rise in voltage (By Uemura, et al., Reports Res. Lab. Asahi Glass Co., Ltd., 55 (2005) pp. 79-82).

In JP 7-8954 A, there has been disclosed a method for cleaning a membrane with impurities deposited thereon, wherein a membrane of an electrolytic water-adjusting apparatus provided with a membrane (an apparatus provided with a membrane, for preparing an alkaline water and an acidic water through electrolyzation of water) is made of material capable of energization when used as a positive anode, at least one of electrolytic water-adjusting electrodes is used as a negative electrode, and an electrolytic cleaning voltage is applied to a membrane between the negative electrode, and the positive electrode, thereby causing elution of impurities such as calcium, and so forth, scaling to the membrane, into water, and effecting cleaning of the membrane.

SUMMARY OF THE INVENTION

The electrolyzer described as above had a problem in that calcium and other trace constituents, contained in the solution introduced into the anode compartment, were precipitated on the membrane to be turned into scales, to thereby induce an increase in the membrane resistance, causing a rise in voltage. This problem is economically undesirable because it will bring about an increase in electricity required for production of polysulfide. Further, it is not easy to remove impurities, such as calcium and so forth, deposited on the membrane, and if an attempt is made to clean impurities with a strong acid, this will raise a possibility that an anode corrosion problem occurs. Those problems are taken as a first issue.

In order to solve the first issue, the invention, in its aspects (1) and (2), provides a method for recovering performance of an electrolyzer for use in production of polysulfide and a method for producing polysulfide.

More specifically, it is an object of the invention to provide a method for recovering performance of an electrolyzer, important to maintain the performance of the electrolyzer over the long term, and a method for producing polysulfide by making use of, or taking advantage of the method for recovering the performance of the electrolyzer, with the use of an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, capable of obtaining polysulfide sulfur in high concentration from an aqueous solution containing sulfide ions by an electrolytic process, particularly capable of producing a digesting liquor containing polysulfide sulfur in high concentration from a white liquor, or a green liquor, in a pulp production process, at a high selectivity, and at high current efficiency, with very low generation of thiosulfate ions as a by-product.

With the method for producing polysulfide with the use of the electrolyzer described as above, a problem has been encountered in that when electrolytic oxidation is started again after stopping-holding of the electrolytic oxidation, a voltage required for electrolytic oxidation becomes higher than before stopping of the electrolytic oxidation. This problem is undesirable because it will bring about an increase in electricity required for production of polysulfide as is the case with the first issue. Those problems are taken as a second issue as follows.

It is another object of the invention to provide, in its aspects (3), and (4), a method for stopping—holding an electrolyzer for use in production of polysulfide, and a method for producing polysulfide, in order to solve the second issue.

More specifically, it is another object of the invention to provide a method for stopping-holding electrolytic oxidation, important to maintain the performance of an electrolyzer for use in production of polysulfide, and a method for producing polysulfide by making use of, or taking advantage of the method for stopping-holding electrolytic oxidation with the use of an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, capable of obtaining polysulfide sulfur in high concentration from an aqueous solution containing sulfide ions in an aqueous solution containing sulfide ions by an electrolytic process, particularly capable of producing a digesting liquor containing polysulfide sulfur in high concentration from a white liquor, or a green liquor, in a pulp production process, at a high selectivity, and at high current efficiency, with very low generation of thiosulfate ions as a by-product.

According to the aspects (1), (2) of the invention, in an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, the first issue is solved by cleaning the anode compartment of the electrolyzer with the use of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent.

The invention provides, in its aspect (1), a method for recovering performance of an electrolyzer, the electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method comprising the step of cleaning the anode compartment of the electrolyzer with the use of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent.

The invention provides, in its aspect (2), a method for producing polysulfide, comprising the steps of introducing a solution containing sulfide ions into an anode compartment of an electrolyzer, the electrolyzer comprising the anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, and introducing an aqueous solution containing caustic soda into the cathode compartment of the electrolyzer, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method further comprising the step of resuming electrolysis after cleaning the anode compartment of the electrolyzer with the use of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent, conducted after stopping the electrolytic oxidation.

The invention according to the aspect (2) is concerned with a method for producing polysulfide either by making use of, or taking advantage of the method for recovering performance of an electrolyzer, according to the aspect (1) of the invention, and it is possible to continue production of polysulfide, with the use of the electrolyzer, by taking the steps of resuming an electrolysis process in the electrolyzer following the process of recovering the performance of the electrolyzer, and repeating those processes.

According to the aspects (3), (4) of the invention, in an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, the second issue is solved by replacing the contents of the anode compartment with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions, and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation.

The invention provides, in its aspect (3), a method for stopping holding an electrolyzer, the electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method comprising the step of replacing the contents of the anode compartment with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions, and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation.

The invention provides, in its aspect (4), a method for producing polysulfide, comprising the steps of introducing a solution containing sulfide ions into an anode compartment of an electrolyzer, the electrolyzer comprising the anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, and introducing an aqueous solution containing caustic soda into the cathode compartment of the electrolyzer, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method further comprising the step of resuming electrolysis after stopping-holding electrolytic oxidation by replacing the contents of the anode compartment with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions, and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation.

The invention according to the aspect (4) is concerned with a method for producing polysulfide either by making use of, or taking advantage of the method for stopping-holding an electrolyzer, according to the aspect (3) of the invention, and it is possible to continue production of polysulfide, with the use of the electrolyzer, by taking the steps of resuming an electrolysis process in the electrolyzer following the process of stopping-holding the electrolyzer, and repeating those processes.

In the present specification, terms “stopping-holding an electrolyzer” and “stopping-holding of electrolytic oxidation”, referred to in connection with the aspects (3), (4) of the invention, mean that when electrolytic oxidation in an electrolyzer is stopped, the electrolytic oxidation is stopped on the predetermined conditions as described, and the electrolytic oxidation as stopped-state is held until resumption of the electrolytic oxidation, and in this connection, the same is applied to similar expression using the term “stopping-holding”.

By virtue of the aspects (1), (2) of the invention, it becomes sufficiently possible to remove scales that are formed due to calcium and other trace constituents being precipitated on the membrane, so that the anode does not undergo dissolution. Accordingly, a voltage required for electrolytic oxidation can be effectively restored (lowered) even if a rise in the voltage occurs over time.

By virtue of the aspects (3), (4) of the invention, it is possible to prevent a voltage required for electrolytic oxidation from rising above that before stopping-holding of electrolytic oxidation when electrolytic oxidation is resumed after stopping-holding of electrolytic oxidation in the electrolyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an electrolyzer, and an anode compartment-cleaning unit, used in Working Examples 1 to 12, and Comparative Examples 1 to 12, respectively;

FIG. 2 is a view showing variation in actual cell voltage required for electrolytic oxidation during electrolytic oxidation according to Comparative Example 13;

FIG. 3 is a view showing variation in actual cell voltage required for electrolytic oxidation during electrolytic oxidation according to Working Example 13;

FIG. 4 is a view showing variation in actual cell voltage required for electrolytic oxidation during electrolytic oxidation according to Working Example 14; and

FIG. 5 is a view showing variation in actual cell voltage required for electrolytic oxidation during electrolytic oxidation according to Comparative Example 14.

BEST MODE FOR CARRYING OUT THE INVENTION

There is no particular limitation to an electrolyzer as a target for application of a method for recovering performance of an electrolyzer, a method for stopping-holding an electrolyzer, and a method for producing polysulfide by making use of, or taking advantage of those methods, according to the invention, if the electrolyzer is an electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein an aqueous solution containing sulfide ions is introduced into the anode compartment, and a polysulfide containing polysulfide sulfur can be obtained through electrolytic oxidation.

The porous anode preferably has a physically continuous three-dimensional mesh structure, a surface thereof being made of nickel, or a nickel alloy containing not less than 50 mass % of nickel, a surface area of the anode being from 500 to 20000 m²/m³ per unit volume of the anode compartment. If the surface area of the anode is less than 500 m²/m³, current density in the surface of the anode will increase, so that not only undesirable by-products such as thiosulfate ions is susceptible to be generated but also anode corrosion caused by nickel is susceptible to occur. If the surface area of the anode exceeds 20000 m²/m³, there arises a possibility of occurrence of a problem from the standpoint of operating an electrolytic process, such as an increase in liquid pressure loss, which is not preferable. Further, the average diameter of openings in the mesh of the anode is preferably in a range of 0.1 to 5 mm.

There is no particular limitation to the aqueous solution containing sulfide ions, introduced into the anode compartment, but the white liquor and green liquor, for use in the process of producing chemical pulp, are suitable for use as the aqueous solution.

As for composition of the white liquor, in the case of the white liquor for use in kraft cooking, as commonly practiced at present, the white liquor normally contains 2 to 6 mol/L of alkaline metal ions, not less than 90 mass % of the alkaline metal ions being sodium ions, and the balance is mostly potassium ions.

Further, the white liquor contains anions including hydroxide ion, sulfide ion, and carbonate ion as main constituents, besides sulfate ion, thiosulfate ion, chloride ion, and sulfite ion. In addition, the white liquor contains trace constituents such as calcium, silicon, aluminum, phosphorous, magnesium, copper, manganese, and iron. That is, the white liquor contains sodium sulfide, and sodium hydroxide as the main constituents thereof.

Meanwhile, as for composition of the green liquor, the green liquor contains sodium sulfide, and sodium carbonate as the main constituents thereof in contrast to sodium sulfide and sodium hydroxide, as the main constituents of the white liquor. Other anions and trace constituents, contained in the green liquor, are the same as those in the case of the white liquor. In the case where the green liquor or the green liquor is fed to the anode compartment of the electrolyzer as the target of application of the method according to the invention to be subjected to electrolytic oxidation, sulfide ion is oxidized, thereby generating polysulfide ions. Further, subsequently, alkaline metal ions are transferred to the cathode compartment through the membrane.

Reaction in the cathode compartment can be variously selected, but reaction whereby hydrogen gas is generated from water is suitably selected. Resultant hydroxide ions as generated are combined with the alkaline metal ions as transferred from the anode compartment to thereby generate alkali hydroxide. A solution introduced into the cathode compartment is preferably, in effect, an aqueous solution of an alkaline metal hydroxide, particularly an aqueous solution of sodium hydroxide, or potassium hydroxide. There is no limitation to concentration of the alkaline metal hydroxide, however, the concentration thereof is, for example, in a range of 1 to 15 mol/L, preferably in a range of 2 to 5 mol/L.

There exists a problem that calcium and other trace constituents, contained in the solution introduced into the anode compartment, are precipitated on the membrane to be turned into scales, to thereby cause an increase in the membrane resistance, to induce a rise in voltage. Since this problem will lead to an increase in electricity required for production of polysulfide, the problem is economically undesirable. Furthermore, if the rise in voltage becomes greater, this will pose the risk of inducing not only an economic problem but also a problem of an increase in by-products such as thiosulfuric acid, sulfuric acid, oxygen, and so forth, leading to a problem such as anode corrosion.

Aspects (1), (2) of the Invention

The present invention has a major feature in that the anode compartment is cleaned as a countermeasure to cope with the problem that a voltage required for electrolytic oxidation will rise over time, due to formation of scales.

There is no particular limitation to an aqueous solution usable for cleaning the anode compartment provided that the aqueous solution contains at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent, and is capable of removing calcium scale, however, use is preferably made of the aqueous solution having low detergency for the anode itself In cases where the anode compartment is cleaned with the use of an aqueous solution having high detergency for the anode, this will cause a surface area of the anode to be reduced, so that current density of the surface of the anode will increase. Such a tendency will become more significant along with an increase in the number of times that cleaning is carried out, so that there arises the risk of inducing the problem of the increase in the by-products such as thiosulfuric acid, sulfuric acid, oxygen, and so forth, leading to the problem such as anode corrosion, and therefore, the aqueous solution having high detergency for the anode is undesirable.

For an inorganic acid usable for cleaning the anode compartment, use is preferably made of hydrochloric acid, and sulfamic acid. In the case of using hydrochloric acid, concentration 0.3 to 1.0 mass % is preferable, and concentration 0.5 to 0.7 mass % is more preferable. In the case of using sulfamic acid, concentration 0.2 to 1.0 mass % is preferable, and concentration 0.3 to 0.5 mass % is more preferable.

An aqueous solution of the inorganic acid in the concentration described has a sufficient power of cleaning calcium scales formed in the electrolyzer used in the present invention, having low detergency for the anode itself If the concentration is excessively low, calcium scales cannot be sufficiently removed, so that this is undesirable, while if the concentration is excessively high, detergency for the anode will increase, so that this is undesirable either.

Further, in the case of using a chelating agent for cleaning the anode compartment, chelating agent concentration 0.5 to 4 mass % is preferable, and concentration 1 to 3 mass % is more preferable. For the chelating agent, use can be made of a chelating agent containing ethylenediamine tetraacetate, hydroxyethyl ethylenediamine triacetate, and so forth, more specifically, such as CLEWAT OH35 (manufactured by Nagase ChemteX Corporation), and so forth, however, there is no particular limitation thereto.

The aqueous solution containing the chelating agent in such concentration as above has a sufficient power of cleaning calcium scales formed in the electrolyzer used in the present invention, having low detergency for the anode itself. If the concentration is excessively low, calcium scales cannot be sufficiently removed, so that this is undesirable while if the concentration is excessively high, detergency for the anode will increase, so that this is undesirable either.

Further, for cleaning of the anode compartment, use can be made of an aqueous solution containing 0.005 to 0.1 mass %, preferably 0.01 to 0.02 mass % of a scale-cleaning agent. For the scale-cleaning agent described, a scale-cleaning agent containing maleic acid base polymer, phosphonic acid, and so forth can be used, including more specifically KURITA QR20 (manufactured by Kurita Water Industries Ltd.), and so forth. However, there is no particular limitation to the scale-cleaning agent provided that the same is usable for cleaning scales to be formed in a process where the white liquor, the green liquor, or a washing water passes during the process of producing, or bleaching the chemical pulp.

The aqueous solution containing the scale-cleaning agent in such concentration as above has a sufficient power of cleaning the calcium scales formed in the electrolyzer used in the present invention, having low detergency for the anode itself. If the concentration is excessively low, calcium scales cannot be sufficiently removed, so that this is undesirable while if the concentration is excessively high, its detergency for the anode will become high, so that this is undesirable either.

Furthermore, for cleaning of the anode compartment, an anticorrosive can be added to the aqueous solution described as above. In the present invention, the anticorrosive refers to chemicals used for prevention of metal from coming into contact with corrosive ions by forming a protective film on the surface of the metal, thereby preventing corrosion of the metal. With the present invention, any of the aqueous solutions used for cleaning of the anode compartment, as described in the foregoing, has low detergency for the anode itself, however, as a result of addition of the anticorrosive, the aqueous solution can gain an advantageous effect such as further decrease in its detergency for the anode.

For the anticorrosive usable in the present invention, use can be made of an anticorrosive containing an organic agent, a surface active agent, an anti-foam agent, and so forth. More specifically, the anticorrosive includes RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.), and so forth, however, there is no particular limitation to the anticorrosive provided that the same is usable in a process where the white liquor, the green liquor, or a washing water passes during the process of producing, or bleaching the chemical pulp. Anticorrosive concentration in a range of 0.05 to 3.0 mass % is preferable.

As a result of addition of the anticorrosive, it is possible to further decrease detergency for the anode, so that concentration of the inorganic acid, the chelating agent, or the scale-cleaning agent, as described in the foregoing, can be rendered higher than that in the case of using each of those chemicals singly, and furthermore, with the addition of the anticorrosive, higher effect of calcium scales removal can be rendered compatible with lower detergency for the anode, thereby enabling more preferable cleaning.

Aspects (3), (4) of the Invention

The electrolyzer as a target for application of the method according to the present invention has had a problem in that, in the case of stopping holding of the electrolytic oxidation, an actual cell voltage upon resuming the electrolytic oxidation will rise above that before the stopping of the electrolytic oxidation. This problem will bring about an increase in the electricity required for production of polysulfide, and is therefore economically undesirable.

Further, if stopping-holding of the electrolytic oxidation, and resumption thereof are repeated, the actual cell voltage will rise by stages. In such a case, there is the risk of causing not only an economic problem as described but also an increase in the by-products such as thiosulfuric acid, sulfuric acid, oxygen, and so forth, leading to the anode corrosion.

The cause of a problem of a rise in the actual cell voltage after the stopping-holding of the electrolytic oxidation, is not clear as yet, however, the problem is presumably attributable to a phenomenon that sulfide ions, and carbonate ions, originating from the solution introduced into the anode compartment, or polysulfide sulfur obtained through the electrolytic oxidation, and so forth, are precipitated as sulfide, or carbonate, on the anode or the membrane during the stopping-holding of the electrolytic oxidation.

Accordingly, in order to get around the problem of the rise in the actual cell voltage after the stopping-holding of the electrolytic oxidation, it is important to lower the concentration of sulfide ions, and carbonate ions, present in the anode compartment during the stopping-holding of the electrolytic oxidation.

To that end, it is naturally effective to replace the solution in the anode compartment with a liquid low in the concentration of sulfide ions, and carbonate ions, however, in such a case, the solution in the anode compartment is preferably replaced with an alkaline aqueous solution low in the concentration of sulfide ions, and carbonate ions. With the present invention, therefore, the solution in the anode compartment is replaced with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions, and not more than 0.1 mass % of carbonate ions. If the solution containing sulfide ions, in the anode compartment, is replaced with an acidic aqueous solution, this will cause generation of hydrogen sulfide, which is undesirable.

There is no particular limitation to alkali contained in the alkaline aqueous solution for use in replacement of the solution in the anode compartment, however, alkali containing 6 to 10 mass % of sodium hydroxide is preferably used. If the solution in the anode compartment is replaced with the alkaline aqueous solution containing sodium hydroxide, this will cause space on both the anode side, and the cathode side of the membrane to be filled up with a liquid of substantially identical composition during the stopping-holding of the electrolytic oxidation, and is therefore preferable.

As for the sodium hydroxide, use is preferably made of sodium hydroxide produced in the cathode compartment during the electrolytic oxidation. In this case, there is obtained an advantage in that it is unnecessary to newly prepare a liquid with which the solution in the anode compartment is replaced, and to newly install a facility into which such a liquid is introduced. Working Examples

The present invention is described in further detail hereinafter with reference to Working Examples, however, it is to be understood that the present invention is not to be limited to those Working Examples.

With Working Examples, and Comparative Examples, calcium concentration and nickel concentration, in a cleaning fluid at the time of completion of cleaning, were measured by the following method.

<Calcium Concentration and Nickel Calcium>

A cleaning fluid sample was diluted to a dilution ratio on the order of 1/500 to 1/50, having thereby quantified calcium ion concentration and nickel ion concentration with the use of an ICP emission spectral analyzer [Vista-MPX, manufactured by Seiko Instruments Inc.]. The calcium ion concentration and the nickel ion concentration, quantified as above, were multiplied by the dilution ratio of the sample to be thereby determined as calcium concentration and nickel concentration, in the cleaning fluid.

[Electrolyzer]

In FIG. 1, there is shown a schematic diagram of an electrolyzer and an anode compartment cleaning unit, including piping and so forth, used in the working examples, and the comparative examples, respectively. In FIG. 1, reference numeral 1 denotes the electrolyzer, shown in longitudinal section. Reference numeral 2 denotes an anode, 3 an anode compartment, 4 a cathode, 5 a cathode compartment, and 6 a membrane. The anode compartment 3 is provided with an anode liquid feed pipe 7 having a valve V1, and an anode liquid discharge pipe 8 having a valve V2. The cathode compartment 5 is provided with a cathode liquid feed pipe 9 having a valve V3, and a cathode liquid discharge pipe 10 having a valve V4. Reference numeral 11 denotes a cleaning fluid tank, 12 a cleaning fluid pump, 13 a cleaning fluid feed pipe, and 14 a cleaning fluid discharge pipe. The electrolyzer 1 is rectangular in cross-sectional shape, and is bilaterally symmetrical with respect to the anode 2.

Those pipes are provided with switching valves V1 to V6, respectively, and various operations such as electrolytic oxidation, stopping thereof, discharge and removal of a polysulfide liquor, supply of a cleaning fluid, circulation, cleaning, discharge removal, and resumption of the electrolytic oxidation are carried out by manipulation of the respective valves. Further, reference numeral 15 denotes a pipe serving as a cleaning-fluid replenishing pipe, doubling as a used-cleaning-fluid discharge pipe, and the pipe is provided with a switching valve V7.

WORKING EXAMPLE 1

A white liquor having the following composition was oxidized by electrolytic oxidation. Electrolysis conditions were as follows: A double-chamber electrolyzer was assembled, the electrolyzer comprising a porous nickel member serving as an anode (an anode surface area per an anode compartment volume: 5600 m²/m³, the average diameter of openings in the mesh of the anode: 0.51 mm, an anode surface area per a unit membrane area: 28 m²/m²), iron expansion metal serving as a cathode, and a perfluoro resin cation-exchange membrane serving as a membrane. The white liquor having the following composition was introduced into the electrolyzer, and electrolysis was carried out under the condition of electrolysis temperature: 85° C., and current density at the membrane: 6 kA/m², having thereby obtained a polysulfide liquor of polysulfide sulfur concentration 9 g/L at current efficiency 95%. Further, NaOH was formed on the cathode side of the electrolyzer at current efficiency 80%, and an aqueous solution containing 10 mass % of NaOH was obtained by adjusting an amount of added water.

<White Liquor Composition>

NaOH concentration; 10.0 mass %, Na₂S concentration; 3.9 mass %, Na₂CO₃ concentration; 3.8 mass %

Electrolytic oxidation was carried out under the electrolysis conditions described as above for 60 days. Voltage at the time of starting the electrolytic oxidation was at 1.498V, and voltage rose over time reaching 1.821V in 60 days. The electrolytic oxidation was stopped, a polysulfide liquor in the anode compartment was removed by discharging, and subsequently, circulatory cleaning was applied to the anode compartment by use of 30 L of hydrochloric acid of 0.7 mass % in concentration as a cleaning fluid, circulating at a flow rate of 460 L/h. The circulatory cleaning was completed in 60 minutes, whereupon the cleaning fluid was discharged to be removed, and subsequently, the electrolytic oxidation was resumed to be followed by operation for 10 days. Table 1 shows calcium concentration (mass ppm), and nickel concentration (mass ppm), in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 1

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that use was made of 0.2 mass % of hydrochloric acid as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 2

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that use was made of 1.1 mass % of hydrochloric acid as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 2

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that the same hydrochloric acid that was used in Working Example 1, with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 3

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 1.0 mass % of hydrochloric acid with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 3

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 1.1 mass % of hydrochloric acid with 0.03 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 4

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 0.4 mass % of an aqueous solution of sulfamic acid was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 4

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 0.1 mass % of an aqueous solution of sulfamic acid was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 5

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that an aqueous solution containing 1.1 mass % of sulfamic acid was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 5

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that the same aqueous solution of sulfamic acid, used in Working Example 4, with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 6

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that an aqueous solution containing 0.8 mass % of sulfamic acid, with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 6

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that an aqueous solution containing 1.1 mass % of sulfamic acid, with 0.03 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 7

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that CLEWAT OH35 (manufactured by Nagase ChemteX Corporation) containing 2.0 mass % of a chelating agent was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 7

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that CLEWAT OH35 (manufactured by Nagase ChemteX Corporation) containing 0.3 mass % of a chelating agent was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 8

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that CLEWAT OH35 (manufactured by Nagase ChemteX Corporation) containing 5.0 mass % of a chelating agent was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 8

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that the same CLEWAT OH35 that was used in Working Example 7 with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 9

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 3.5 mass % of CLEWAT OH35 with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 9

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 1, except that 5.0 mass % of CLEWAT OH35 with 0.03 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 10

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used in Working Example 1, except that use was made of 0.015 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 10

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used in Working Example 1, except that use was made of 0.003 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 11

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used in Working Example 1, except that use was made of 0.12 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 11

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used for Working Example 1, except that KURITA QR20 (manufactured by Kurita Water Industries Ltd.) used in Working Example 10, with 0.5 mass % of RESCOR A-825 as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 12

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used for Working Example 1, except that 0.1 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) with 0.5 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 12

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those used for Working Example 1, except that 0.12 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) with 0.03 mass % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and after stopping the electrolytic oxidation, cleaning was applied, subsequently, resuming electrolytic oxidation. Table 1 shows calcium concentration, and nickel concentration in the cleaning fluid at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

As shown in Table 1, it is evident from comparison of Working Examples with Comparative Examples that, in cases where hydrochloric acid, sulfamic acid, a chelating agent such as CLEWAT OH35, and a scale-cleaning agent such as KURITA QR20 (hereinafter those are referred to as a cleaning fluid), in concentration within a range according to the invention, respectively, are used (Working Examples 1, 4, 7, 10), calcium concentration after completion of cleaning was found at not less than 90 mass ppm, indicating sufficient dissolution of calcium scales while nickel concentration after completion of cleaning was found at not more than 220 mass ppm, indicating low detergency for an anode.

Further, in cases where those cleaning fluids each are used in an adequate concentration with 0.5 mass % of RESCOR A-825 as an anticorrosive, added thereto, (Working Examples 2, 3, 5, 6, 8, 9, 11, 12), calcium concentration after completion of cleaning was found substantially at the same level as that in the case where RESCOR A-825 was not added, however, nickel concentration after completion of cleaning was found at not more than 60 mass ppm, indicating that detergency for an anode was under control in spite of sufficient dissolution of calcium scales.

Meanwhile, if cleaning fluid concentration is excessively low (Comparative Examples 1, 4, 7, 10), excessively high (Comparative Examples 2, 5, 8, 11), or concentration of RESCOR A-825 added to a cleaning fluid is 0.03 mass % (Comparative Examples 3, 6, 9, 12), calcium concentration after completion of cleaning was found at not less than 60 mass %, or nickel concentration after completion of cleaning was found at not less than 270 mass ppm, indicating that a cleaning method is undesirable in either case.

TABLE 1 actual cell voltage required for Ca conc. of Ni conc. of electrolytic cleaning cleaning oxidation 10 days fluid after fluid after after cleaning fluid cleaning cleaning resumption of (% = mass %) (mass ppm) (mass ppm) electrolysis (V) working hydrochloric acid (0.7%) 133 187 1.507 example 1 compara. hydrochloric acid (0.2%) 32 124 1.724 example 1 compara. hydrochloric acid (1.1%) 144 356 1.490 example 2 working hydrochloric acid (0.7%) 131 35 1.506 example 2 RESCOR A-825 (0.5%) working hydrochloric acid (1.0%) 138 52 1.502 example 3 RESCOR A-825 (0.5%) compara. hydrochloric acid (1.1%) 142 319 1.495 example 3 RESCOR A-825 (0.03%) working sulfamic acid (0.4%) 112 214 1.528 example 4 compara. sulfamic acid (0.1%) 27 99 1.706 example 4 compara. sulfamic acid (1.1%) 121 386 1.517 example 5 working sulfamic acid (0.4%) 110 53 1.537 example 5 RESCOR A-825 (0.5%) working sulfamic acid (0.8%) 115 60 1.532 example 6 RESCOR A-825 (0.5%) compara. sulfamic acid (1.1%) 119 352 1.530 example 6 RESCOR A-825 (0.03%) working CLEWAT OH35 (2.0%) 118 138 1.533 example 7 compara. CLEWAT OH35 (0.3%) 55 57 1.738 example 7 compara. CLEWAT OH35 (5.0%) 122 306 1.519 example 8 working CLEWAT OH35 (2.0%) 114 29 1.535 example 8 RESCOR A-825 (0.5%) working CLEWAT OH35 3.5% 117 23 1.531 example 9 RESCOR A-825 0.5% compara. CLEWAT OH35 (5.0%) 120 301 1.514 example 9 RESCOR A-825 (0.03%) working QR20 (0.015%) 92 128 1.555 example 10 compara. QR20 (0.003%) 19 97 1.692 example 10 compara. QR20 (0.12%) 96 281 1.550 example 11 working QR20 (0.015%) 89 10 1.559 example 11 RESCOR A-825 (0.5%) working QR20 (0.1%) 93 16 1.557 example 12 RESCOR A-825 0.5% compara. QR20 (0.12%) 94 274 1.556 example 12 RESCOR A-825 (0.3%)

WORKING EXAMPLE 13

A white liquor having the same composition as that of the white liquor for Working Example 1 was subjected to electrolytic oxidation under the same condition and in the same electrolyzer, as in the case of Working Example 1, for 60 days. Immediately after stopping the electrolytic oxidation, a caustic soda aqueous solution having the following composition, as an alkaline aqueous solution, was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

[Caustic Soda Aqueous Solution Composition]

NaOH concentration; 9.0 mass %; sulfide ion concentration; 0.2 mass %, carbonate ion concentration; 0.2 mass %

The alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, to be followed by cleaning of the anode compartment under the same conduction as in the case of Working Example 1, with the use of 0.7 wt % of hydrochloric acid as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 14

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 0.7 wt % of hydrochloric acid with 0.5 wt % of RESCOR A-825 (manufactured by Toei Kasei Co., Ltd.) as an anticorrosive, added thereto, was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 15

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 1.0 wt % of hydrochloric acid with 0.5 wt % of RESCOR A-825 as an anticorrosive, added thereto, was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 16

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and an aqueous solution of 0.4 wt % sulfamic acid was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 17

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and an aqueous solution of 0.8 wt % sulfamic acid with 0.5 wt % of RESCOR A-825 as an anticorrosive, added thereto, was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 18

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 2.0 wt % of CLEWAT OH35 (manufactured by Nagase ChemteX Corporation) was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 19

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 3.5 wt % of CLEWAT OH35 with 0.5 wt % of RESCOR A-825 as an anticorrosive, added thereto, was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 20

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 0.015 mass % of KURITA QR20 (manufactured by Kurita Water Industries Ltd.) was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

WORKING EXAMPLE 21

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, and after stopping the electrolytic oxidation, a caustic soda aqueous solution was fed into the anode compartment, having stopped feeding of the aqueous solution after liquid composition at an outlet of the anode compartment was found the same as that at an inlet thereof.

The anode compartment was cleaned under the same conduction as in the case of Working Example 1 except that the alkaline aqueous solution in the anode compartment was discharged after stopping-holding the electrolyzer for 24 hours, and 0.1 wt % of KURITA QR20 with 0.5 wt % of RESCOR A-825 as an anticorrosive, added thereto, was used as a cleaning fluid, and electrolytic oxidation was resumed after discharge and removal of the cleaning fluid, continuing operation for 10 days. Table 2 shows calcium concentration, and nickel concentration, in the cleaning fluid, at the time of completion of cleaning, and a voltage required for electrolytic oxidation 10 days after resumption of electrolysis.

COMPARATIVE EXAMPLE 13

Electrolytic oxidation was conducted under a condition of the same electrolyzer, and white liquor as those for Working Example 13, after stopping the electrolytic oxidation, operation was stopped, and held for 24 hours with the anode compartment kept in such a state as filled with the white liquor, and a polysulfide liquor, and the electrolytic oxidation was resumed with the anode compartment kept as it is without any cleaning, continuing operation for 10 days. A voltage required for electrolytic oxidation 10 days after resumption of electrolysis is shown in Table 2.

As shown in Table 2, it is evident from comparison of Working Examples with Comparative Example that, in the case where after replacement of the contents of the anode compartment with a cathode liquor of polysulfide ion concentration, and carbonate ion concentration, at 0.1 mass %, use was made of the cleaning fluid in concentration in the range according to the invention (Working Examples 13, 16, 18, 20), calcium concentration after completion of the cleaning was found at not less than 90 mass %, indicating that calcium scales were sufficiently dissolved, and the voltage required for electrolytic oxidation, in 10 days after the resumption of electrolysis, was recovered to a value of thereof, close to the voltage upon the start of electrolysis while nickel concentration after completion of the cleaning was found at not more than 220 mass %, indicating that detergency for the anode was low.

Further, while in the case where use was made of any of those cleaning fluids, with 0.5 wt % of the anticorrosive RESCOR A-825 added thereto (Working Examples 14, 15, 17, 19, 21), calcium concentration after completion of the cleaning was found substantially the same as that in the case where RESCOR A-825 was not added, nickel concentration after completion of the cleaning was found at not more than 60 mass %, indicating that detergency for an anode was under control while calcium scales were sufficiently dissolved.

In contrast, in the case (Comparative Example 13) where any treatment and cleaning were not applied to the electrolyzer after stopping the electrolytic oxidation, it is evident that the voltage required for electrolytic oxidation, after the resumption of electrolysis, was higher than that before stopping the electrolytic oxidation.

TABLE 2 actual cell voltage required Ca conc. of for electrolytic cleaning Ni conc. of oxidation 10 fluid after cleaning fluid days after cleaning fluid cleaning after cleaning resumption of (% = mass %) (mass ppm) (mass ppm) electrolysis (V) working example 13 hydrochloric acid (0.7%) 123 193 1.511 working example 14 hydrochloric acid (0.7%) 125 38 1.502 RESCOR A-825 (0.5%) working example 15 hydrochloric acid (1.0%) 140 50 1.488 RESCOR A-825 (0.5%) working example 16 sulfamic acid (0.4%) 108 211 1.533 working example 17 sulfamic acid (0.8%) 121 56 1.529 RESCOR A-825 (0.5%) working example 18 CLEWAT OH35 2.0% 115 59 1.536 working example 19 CLEWAT OH35 (3.5%) 118 32 1.527 RESCOR A-825 (0.5%) working example 20 QR20 (0.015%) 96 34 1.541 working example 21 QR20 (0.1%) 100 29 1.535 RESCOR A-825 (0.5%) compara. example 13 — — — 1.874

COMPARATIVE EXAMPLE 14

A white liquor having the following composition was oxidized by electrolytic oxidation. Electrolysis conditions were as follows: A double-chamber electrolyzer was assembled, the electrolyzer comprising a porous nickel member serving as an anode (an anode surface area per an anode compartment volume: 5600 m²/m³, the average diameter of openings in the mesh of the anode: 0.51 mm, an anode surface area per a unit membrane area: 28 m²/m²), iron expansion metal serving as a cathode, and a fluororesin cation-exchange membrane serving as a membrane. The white liquor having the following composition was introduced into the electrolyzer, and electrolysis was carried out under the condition of electrolysis temperature: 85° C., and current density at the membrane: 6 kA/m², having thereby obtained a polysulfide liquor of 9 g/L in polysulfide sulfur concentration at current efficiency 95%. Further, NaOH was formed on the cathode side of the electrolyzer at current efficiency 80%, and an aqueous solution containing 10 mass % of NaOH was obtained by adjusting an amount of added water.

<White Liquor Composition>

NaOH concentration; 10.0 mass %, Na₂S concentration; 3.9 mass %, Na₂CO₃ concentration; 3.8 mass %

After operating the electrolyzer under the electrolysis conditions described as above for 19 days, electrolytic oxidation was stopped. After the electrolyzer with the anode compartment kept in such a state as filled up with the white liquor (containing polysulfide) present upon stopping of the electrolytic oxidation, as it is, was stopped and held for 24 hours, the electrolytic oxidation was resumed, and operation was continued for further 10 days. Variation in actual cell voltage required for electrolytic oxidation during the electrolytic oxidation is shown in FIG. 2.

WORKING EXAMPLE 22

A white liquor having the same composition as that for Comparative Example 14 was oxidized by electrolytic oxidation by operating the electrolyzer under the same conditions as those in the case of Comparative Example 14, for 20 days, and subsequently, the electrolytic oxidation was stopped. Immediately after stopping the electrolytic oxidation, a cathode liquor having the following composition, as an alkaline aqueous solution, was fed into the anode compartment, having stopped feeding of the liquor after liquor composition at an outlet of the anode compartment was found the same as that at an inlet thereof. After stopping-holding the electrolyzer for 24 hours, the contents of the anode compartment were replaced with the white liquor, and the electrolytic oxidation was resumed, continuing operation for further 10 days. Variation in actual cell voltage required for electrolytic oxidation during the electrolytic oxidation is shown in FIG. 3.

[Cathode Liquor Composition]

NaOH concentration; 9.9 mass %, sulfide ion concentration; less than 5 mass %, cabonate ion concentration; less than 5 mass %

WORKING EXAMPLE 23

A white liquor having the same composition as that for Comparative Example 14 was oxidized by electrolytic oxidation by operating the electrolyzer under the same conditions as those in the case of Comparative Example 14, for 21 days, and subsequently, the electrolytic oxidation was stopped. Immediately after stopping the electrolytic oxidation, a caustic soda aqueous solution having the following composition, as an alkaline aqueous solution, was fed into the anode compartment, having stopped feeding of liquor after liquor composition at an outlet of the anode compartment was found the same as that at an inlet thereof. After stopping holding the electrolyzer for 24 hours, the contents of the anode compartment were replaced with the white liquor, and the electrolytic oxidation was resumed, continuing operation for further 9 days. Variation in actual cell voltage required for electrolytic oxidation during the electrolytic oxidation is shown in FIG. 4.

[Caustic Soda Aqueous Solution Composition]

NaOH concentration; 9.0 mass %, sulfide ion concentration; 0.05 mass %, cabonate ion concentration; 0.05 mass %

COMPARATIVE EXAMPLE 15

A white liquor having the same composition as that for Comparative Example 14 was oxidized by electrolytic oxidation by operating the electrolyzer under the same conditions as those in the case of Comparative Example 14, for 20 days, and subsequently, the electrolytic oxidation was stopped. Immediately after stopping the electrolytic oxidation, a caustic soda aqueous solution having the following composition, as an alkaline aqueous solution, was fed into the anode compartment, having stopped feeding of liquor after liquor composition at an outlet of the anode compartment was found the same as that at an inlet thereof. After stopping-holding the electrolyzer for 24 hours, the contents of the anode compartment were replaced with the white liquor, and the electrolytic oxidation was resumed, continuing operation for further 10 days. Variation in actual cell voltage required for electrolytic oxidation during the electrolytic oxidation is shown in FIG. 5.

[Caustic Soda Aqueous Solution Composition]

NaOH concentration: 9.0 mass %, sulfide ion concentration: 0.2 mass %, cabonate ion concentration: 0.2 mass %

It is evident from comparison of Working Examples with Comparative Examples with reference to FIGS. 2 to 5 that the voltage required for electrolytic oxidation, after the resumption of electrolysis, was higher than that before stopping the electrolytic oxidation in the case (Comparative Example 14) where any treatment was not applied to the electrolyzer after stopping the electrolytic oxidation, or in the case (Comparative Example 15) where the contents of the anode compartment were replaced with such a caustic soda aqueous solution as one having 0.2 mass % of sulfide ion concentration and cabonate ion concentration, respectively, upon stopping the electrolytic oxidation. In contrast, it is evident that the actual cell voltage required for electrolytic oxidation after the resumption of electrolysis underwent little difference from the voltage before stoppage of the electrolytic oxidation, in the case (Working Example 22) where the contents of the anode compartment were replaced with a cathode liquor of not more than 0.1 mass % of sulfide ion concentration and cabonate ion concentration, respectively, upon stopping the electrolytic oxidation, or in the case (Working Example 23) where the contents of the anode compartment were replaced with a caustic soda aqueous solution of not more than 0.1 mass % of sulfide ion concentration and cabonate ion concentration, respectively, upon stopping the electrolytic oxidation. 

1. A method for recovering performance of an electrolyzer, the electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method comprising the step of cleaning the anode compartment of the electrolyzer with the use of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent.
 2. The method of claim 1, wherein the aqueous solution for use in cleaning the anode compartment contains hydrochloric acid having concentration 0.3 to 1.0 mass %.
 3. The method of claim 1, wherein the aqueous solution for use in cleaning the anode compartment contains sulfamic acid having concentration in a range of 0.2 to 1.0 mass %.
 4. The method of claim 1, wherein the aqueous solution for use in cleaning the anode compartment contains the chelating agent having concentration in a range of 0.5 to 4 mass %.
 5. The method of claim 1, wherein the aqueous solution for use in cleaning the anode compartment contains the scale-cleaning agent having concentration in a range of 0.005 to 0.1 mass %.
 6. The method of claim 1, wherein the aqueous solution for use in cleaning the anode compartment contains anticorrosive having concentration in a range of 0.05 to 3.0 mass %.
 7. The method of claim 1, wherein the contents of the anode compartment are replaced with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation, subsequently the anode compartment is cleaned with the use of an aqueous solution containing at least either one of the inorganic acid, the chelating agent, and the scale-cleaning agent.
 8. A method for producing polysulfide, comprising the steps of introducing a solution containing sulfide ions into an anode compartment of an electrolyzer, the electrolyzer comprising the anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, and introducing an aqueous solution containing caustic soda into the cathode compartment of the electrolyzer, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method further comprising the step of resuming electrolysis after cleaning the anode compartment of the electrolyzer with the use of an aqueous solution containing at least either one of an inorganic acid, a chelating agent, and a scale-cleaning agent, conducted after stopping the electrolytic oxidation.
 9. The method of claim 8, wherein the contents of the anode compartment are replaced with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation, thereby stopping-holding electrolysis.
 10. A method for stopping-holding an electrolyzer, the electrolyzer comprising an anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, wherein a solution containing sulfide ions is introduced into the anode compartment, and an aqueous solution containing caustic soda is introduced into the cathode compartment, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, wherein the contents of the anode compartment are replaced with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation, thereby stopping-holding electrolysis.
 11. The method of claim 10, wherein the alkaline aqueous solution for use in replacement of the solution in the anode compartment contains caustic soda having concentration in the range of 6 to 10 mass %.
 12. The method of claim 10, wherein alkaline aqueous solution for use in replacement of the solution in the anode compartment contains an aqueous solution including caustic soda produced in the cathode compartment.
 13. A method for producing polysulfide, comprising the steps of introducing a solution containing sulfide ions into an anode compartment of an electrolyzer, the electrolyzer comprising the anode compartment provided with porous anodes, a cathode compartment, and a membrane providing a partition between the anode compartment and the cathode compartment, and introducing an aqueous solution containing caustic soda into the cathode compartment of the electrolyzer, thereby producing a polysulfide containing polysulfide sulfur through electrolytic oxidation, said method further comprising the step of resuming electrolysis after stopping-holding electrolytic oxidation by replacing the contents of the anode compartment with an alkaline aqueous solution containing not more than 0.1 mass % of sulfide ions, and not more than 0.1 mass % of carbonate ions upon stopping the electrolytic oxidation. 